Intermittent hot water one SIME combi boiler?

I have a sime combi boiler, sorry, I dont know where to find the model number, but the heating (radiators) are working fine. I can obtainhot water when the heating system is on (radiators are finetoo),but when I switch the heating off then hot wasappearsto beintermitent.the pressure is set at 1.5bar, ofwhich I have increased to 2bar to see if this would rid of the fault butto no avail,hence reduced back to 1.5bar. The boilr though has been working at around 0.6 to 0.8 bar for almost 2years as I did not know the required pressure required until recentlyhaving a boiler engineer visit. The engineer increased the 0.6bar to 1.5 bar and it appeared to work for a time, I have since called hime back and he increased to 2bar and then appeared to work finefor a while. Again had him visit and he has reduced the pressue to 1.5bar and again worked for while but now hot water unless I have the central heating on.
The engineer says that it is going to be expensive as the parts will be replaced on a 'hit and miss' basis. Thissounds expensive and would appreciate a response if anyone has experienced the same symptons as above.
Many thanks in advance, best regards